1. Field of the Invention
The present invention relates to a method of and an apparatus for forming a bent sheet of glass such as an automobile front windshield or the like.
2. Description of the Prior Art
For manufacturing a bent sheet of glass such as an automobile front windshield, for example, it has been customary to heat a sheet of glass up to a temperature near its softening point while it is being fed by horizontal rolls in a heating furnace, and then to transfer the heated sheet of glass onto a ring mold that is positioned below an upper mold having a fully continuous molding surface. The heated sheet of glass is thereafter pressed to a desired bent configuration between the ring mold and the upper mold.
During the pressing of the heated sheet of glass, the ring mold is held in contact with the peripheral edge of the glass sheet for a relatively long period of time. Therefore, plane compressive stresses are developed in the peripheral edge of the glass sheet by the ring mold, and hence plane tensile stresses are also developed in the region of the glass sheet inward of the peripheral edge thereof to equilibrate the plane compressive stresses developed in the peripheral edge. These plane stresses are different from stresses that are produced transversely in the glass sheet when it is tempered. If the plane stresses in the glass sheet are unduly large, then the glass sheet will tend to break when it is set in an automobile windshield frame or subjected to other stresses after set in an automobile windshield frame.
Japanese laid-open patent publication No. 52-78226 discloses a proposal for keeping plane compressive stresses and plane tensile stresses within a predetermined range in a laminated sheet of glass for use as an automobile front windshield.
However, it has been quite difficult to maintain, within a predetermined range, plane compressive stresses which are developed in the peripheral edge of a sheet of glass that is held in contact with a ring mold for a long period of time. Even if the developed plane compressive stresses can successfully be held in the predetermined range, they are liable to cause the glass sheet to shatter under certain conditions.